1/2" on top of 1/2 or 5/8 seems to be the ticket. I Used 1/2" Screw homasote to plywood and make sure anything on top is not screwed in further than your homasote (no noise/vibration transfer across the 2 media). Some glue, but if you want to cut in to place a structure, it is easier than chipping out glued pieces from the plywood board.
As for cutting. The sawing method above works, but I used a dermal saw-Max with a vacuum in the other hand. No muss, no fuss.
If you do a thorough search on this subject you could write a college dissertation and come out with more questions than you posted here. It is a somewhat common subject. I think you will be glad you are suing the homasote.
